Flexographic printing is widely used in the production of newspapers and in the decorative printing of packaging media. Photosensitive printing plates have been developed to meet the demand for fast, inexpensive processing and long press runs.
The photocurable element generally comprises a substrate, one or more photocurable layers, and a protective cover sheet. The protective cover sheet is formed from plastic or any other removable material that can protect the plate or photocurable element from damage until it is ready for use. The photocurable element can optionally comprise a slip film disposed between the protective cover sheet and the photocurable layer(s) to protect the plate from contamination, increase ease of handling, and act as an ink-accepting layer.
The photosensitive resin composition is deposited onto the substrate to form a printing element. The photosensitive resin composition may be deposited onto the substrate in a variety of ways, e.g., by extrusion, roll coating, heat processing, solvent casting, and the like. These techniques can be readily carried out by those skilled in the art.
The desired image is produced on the printing plate by exposing selected portions of the photosensitive resin to actinic radiation. Selective exposure of the photosensitive resin can be achieved for example, by the use of an image-bearing transparency such as a negative film on the surface of the photosensitive layer, through the front side of the photosensitive resin. Areas of the transparency opaque to actinic radiation prevent the initiation of free-radical polymerization within the photosensitive layer directly beneath the transparency. Transparent areas of the image-bearing element allow the penetration of actinic radiation into the photosensitive layer, initiating free-radical polymerization, rendering those areas insoluble in the processing solvent. Alternatively, exposure of selected portions of the photosensitive layer to laser radiation may also initiate free-radical polymerization, rendering those areas insoluble in the processing solvent.
The unexposed and therefore unhardened portions of the resin are selectively removed by washing in a suitable solvent. Washing may be accomplished by a variety of processes, including brushing, spraying, or immersion. The resulting surface has a relief pattern that reproduces the image to be printed. The printing element is mounted on a press and printing commences.
The support sheet, or backing layer, can be formed from a suitable transparent or opaque material such as paper, cellulose film, plastic, or metal. Preferred substrate materials include steel, copper or aluminum sheets, plates, or foils, paper, or films or sheets made from synthetic polymeric materials such as polyesters, polystyrene, polyolefins, polyamides, and the like.
The photosensitive layer(s) can include a variety of known photosensitive materials, such as photopolymers, initiators, reactive diluents, fillers, and dyes. Preferred photosensitive compositions include an elastomer compound, an ethylenically unsaturated compound having at least one terminal ethylenic group, and a photoinitiator. Such materials are described in numerous patents and publications and are well known to those skilled in the art
Normally, these photopolymer resins have very poor adhesive properties to the underlying metal or polymer substrates. Therefore, the support sheet can optionally comprise an adhesive layer, i.e., a primer or anchor layer, for more secure attachment to the photocurable layer(s). However, the solvent-like components in the photopolymer resin constantly attack the primer layer before and after curing, resulting in the delamination of the cured relief photopolymer from the underlying substrate.
Water-based polyurethane coatings with various kinds of inorganic fillers have been suggested for use as the primer layer in flexographic printing plates. These fillers include small particles of titanium oxide, zinc oxide, calcium carbonate, silicon gels (zeospheres and zeolites), and polymer materials, such as small particles of polyamides, polystyrene, and other similar materials.
While the water-based polyurethanes generally have very good resistance to water and/or organic solvents after the coatings are cured, when these coatings are used as primer layers for flexographic printing plates, the water/solvent resistance properties are weakened by the continuous intersurface contact either on top or at the bottom of the primer layer.
U.S. Pat. No. 2,760,863 to Plambeck, the subject matter of which is herein incorporated by reference in its entirety, teaches that when the support material is not sufficiently adherent to the photopolymerized layer, a separate anchor layer may be used. The anchor layer is made of a compatible resin or film-forming polymer which is strongly adherent to both the support and the photopolymerized layer. Plambeck discloses that in some cases, two or more different anchor layers can be used so that the photopolymerized layer is strongly adherent to the coated support.
U.S. Pat. No. 6,551,759 to Daems et al., the subject matter of which is herein incorporated by reference in its entirety, describes an optional primer layer provided between the support layer and the photopolymerizable layer. The primer layer preferably comprises aziridine functional compounds which may be diluted with solids that comprise inert compatible polymeric organic binders, coupling agents, particulate, comonomers, other priming agents, and the like. U.S. Pat. No. 5,187,044 to Prioleau et al., the subject matter of which is herein incorporated by reference in its entirety, also describes a primer layer that comprises an aziridine functional material that may be diluted with solids that comprise inert compatible polymeric organic binders, coupling agents, particulate, comonomers, other priming agents, and the like. Prioleau et al. suggests that the primer compositions are suited for use in flexographic printing plates when used in conjunction with a support layer or sheet to which they are adhered.
U.S. Pat. No. 6,410,635 to Kaylo et al., the subject matter of which is herein incorporated by reference in its entirety, describes compositions comprising an exfoliated silicate material (i.e., a montmorillonite clay) to enhance properties such as appearance, crater resistance, and rheology control. Kaylo et al. suggest that the compositions are particularly useful as color and/or clear coatings in color-clear composite coatings. However, Kaylo et al. do not suggest that the compositions are usable as the primer or anchor layer for a flexographic printing plate.
U.S. Pat. No. 3,036,913 to Burg and U.S. Pat. No. 5,175,076 to Ishikawa et al., the subject matter of each of which is herein incorporated by reference in its entirety also describe primer compositions for increasing the adhesion of photopolymerizable layers to an underlying substrate in processes for making printing plates.
While many primer compositions have been developed, there remains a need in the art for further improvements to the primer layers used for adhering photopolymer resins to an underlying substrate in the manufacture of flexographic printing plates.
The inventors have surprisingly discovered that the addition of a layered silicate material as a filler, more preferably a montmorillonite clay, into a water-based polyurethane primer composition, produces an improved primer composition with a stronger water/solvent resistance and less delamination as compared to polyurethane primers of the prior art.